It's ATi who really have the confusing numbering system:
9100 = 8500
9100SE = castrated 9100
9200 = castrated 9100
Then you have all the Pro/XT/SE variants of all the cards.
The GeForce 4 MX has a Dx7 feature set, and is faster than, while having the same feature set as, the geForce 2s.
The GeForce 4 Ti has a Dx 8.0 feature set
The 5xxx series is Dx 9.0 ranging from slow (5200) to fast (5950)
The 6xxx series adds the LE (Slow) GT (faster) and Ultra (fastest) monnikers to allow more products. All 6xxx cards are Dx 9.0c parts.
The ATi naming to my mind is confusing. An XT or a Pro both sound good, but IIRC one or other of them is the slower part.... Ultra sounds like the dogs bollox